<!DOCTYPE html>

<html>

<head lang="en">
  <meta charset="UTF-8">
  <title></title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    table {
      border-collapse: collapse;
      border-spacing: 0;
      border: 1px solid #c0c0c0;
      width: 500px;
      margin: 100px auto;
      text-align: center;
    }

    th {
      background-color: #09c;
      font: bold 16px "微软雅黑";
      color: #fff;
      height: 24px;
    }

    td {
      border: 1px solid #d0d0d0;
      color: #404060;
      padding: 10px;
    }

    .allCheck {
      width: 80px;
    }
  </style>
</head>

<body>
  <table>
    <tr>
      <th class="allCheck">
        <input type="checkbox" name="" id="checkAll"> <span class="all">全选</span>
      </th>
      <th>商品</th>
      <th>商家</th>
      <th>价格</th>
    </tr>
    <tr>
      <td>
        <input type="checkbox" name="check" class="ck">
      </td>
      <td>小米手机</td>
      <td>小米</td>
      <td>￥1999</td>
    </tr>
    <tr>
      <td>
        <input type="checkbox" name="check" class="ck">
      </td>
      <td>小米净水器</td>
      <td>小米</td>
      <td>￥4999</td>
    </tr>
    <tr>
      <td>
        <input type="checkbox" name="check" class="ck">
      </td>
      <td>小米电视</td>
      <td>小米</td>
      <td>￥5999</td>
    </tr>
  </table>
  <script>
    // 获取元素
    const checkAll = document.querySelector('#checkAll')
    const cks = document.querySelectorAll('.ck')

    checkAll.addEventListener('click', function () {
      // 我们可以拿到大按钮状态，并且赋值给3个小按钮
      for (let [i, e] of cks.entries()) {
        e.checked = checkAll.checked
      }
    })

    // 判断小按钮被选中的个数  是不是等于3  等于3 大的就选中 否则不选
    for (let ck of cks) {
      // 给每一个ck绑定点击事件
      ck.addEventListener('click', function () {
        // document.querySelectorALl(.ck: checked').length·===3这句话是不是在比较?true false
        // console.log(document.querySelectorAll('.ck:checked').length === 3);
        checkAll.checked = document.querySelectorAll('.ck:checked').length === cks.length
      })
    }
  </script>
</body>

</html>